Whisper's mod and other mods like it automate removing almost everything that isn't part of the precombines, but unfortunately a lot of the trash and debris in many settlements is part of the vanilla precombine data.

If any other mod touches any of the precombine/previs data within that cell, the precombine and previs is disabled for that cell automatically. If I remember right, you can only have one mod that changes a given cell's precombine data in your load order. Yeap, Immersive Cleaning uses re-done precombines from what I can tell, which would overwrite those from Boston FPS Fix and thus break the precombines just like Scrap Everything does.